After creating a a png in Fireworks 4 with several graphics ,text etc. and saving it . a few weeks later I go back to edit it , it won't let me. It is not group /ungroup. Is there a time limit to edit or is there a setting. It is very flustrating !!...

Fireworks 4 ,not able to edit PNG

After creating a a png in Fireworks 4 with several graphics ,text etc. and saving it . a few weeks later I go back to edit it , it won't let me. It is not group /ungroup. Is there a time limit to edit or is there a setting. It is very flustrating !!